1. Field of the Invention
The present invention relates to a photographing lens unit and an imaging apparatus with the same, and more specifically, to a photographing lens unit including a plurality of lenses and an imaging apparatus in which a camera module with such a photographing lens unit is incorporated.
2. Description of the Background Art
In recent years, the demands for downsizing, reduction in weight, reduction in cost, and enhancement in image quality have been crucial to a photographing lens unit such as a mobile phone camera and digital camera, and particularly, in a photographing lens unit of a mobile phone camera, these demands have become stronger and stronger. On the other hand, in a mobile phone camera, the demand for high performance as a camera is also strong, and a camera module having an imaging element of 3 megapixels or more has been introduced.
The camera of 3 megapixels or more, even if it is a mobile phone camera, has performance similar to that of a thin digital camera. When such a high performance camera is mounted on a mobile phone, an optical part, such as an adjustable diaphragm and an ND (Neutral Density) filter used in a digital camera, is required in the photographing lens unit.
As a configuration of the photographing lens unit of 3 megapixels or more, about three or four lenses are often used. As the configuration of such a photographing lens unit, the following have been devised.
A photographing lens system described in Japanese Patent Laying-Open No. 03-288811 includes a plurality of plastic lenses arrayed on an optical axis, and one glass lens arranged in an intermediate position of the plurality of plastic lenses. Among the photographing lenses, only the glass lens is provided with an image formation action.
A wide angle lens system described in Japanese Patent Laying-Open No. 08-262322 is constituted by a first lens having negative refractive power, a second lens having positive refractive power, and third and fourth lenses each made of synthetic resin and having positive refractive power in order from the object side. These compound lenses are configured so that a synthetic focal distance and an Abbe number thereof satisfy predetermined conditions.
A photographing lens system described in Japanese Patent Laying-Open No. 09-258100 is constituted by four single lenses of a first biconvex lens, a second biconcave lens with a strong concave surface facing the object side, a positive third lens with a strong convex surface facing the image side, and a fourth lens with a strong convex surface facing the object side in order from the object side.
An imaging lens system described in Japanese Patent Laying-Open No. 11-038316 is constituted by a first lens having a negative refractive index with a concave surface facing the object side, and a subsequent lens group in order from the object side. A first lens group satisfies a predetermined condition equation with respect to a curvature radius, and the subsequent lens group has at least one negative lens element.
A photographing lens system described in Japanese Patent Laying-Open No. 2002-228922 is constituted by a first lens group of a first lens, which is a positive lens, and a second lens, which is a negative lens, a second lens group of a third lens, which is a positive lens with at least one refractive surface formed into an aspherical shape, and a third lens group of a fourth lens, which is a negative lens with at least one refractive surface formed into an aspherical shape, in order from the object side.
A photographing lens system described in Japanese Patent Laying-Open No. 2003-255222 includes a first lens group of a first lens having positive refractive power and a second lens having negative refractive power, a second lens group including a meniscus lens with at least one refractive surface formed into an aspherical shape and with a concave surface facing the object side, and an aspherical third lens group.
A wide angle monofocal lens system described in Japanese Patent Laying-Open No. 2004-246166 includes a first lens having a meniscus shape with a concave surface facing the object side, a second lens having positive power with a concave-shaped surface on the object side, a third lens having a meniscus shape with a convex surface facing the image surface side, and a fourth lens having a meniscus shape with a concave surface facing the image surface side, in order from the object side.
The use of an optical part such as an adjustable diaphragm and an ND filter in a photographing lens unit makes it difficult to realize downsizing as a whole and a favorable optical property.
For example, in the case where the adjustable diaphragm and the ND filter are arranged on the object side of the photographing lens unit, an entire length obtained by adding the thicknesses of the adjustable diaphragm and the ND filter to the entire length of the photographing lens unit is the entire length as a camera module. Therefore, even when the entire length of the photographing lens unit is short, the entire length as the camera module becomes long.
In the case where the adjustable diaphragm and the ND filter are arranged on the image side of the photographing lens unit, a back focal distance of the photographing lens unit corresponding to a distance from a lens rear end portion to an imaging surface becomes long, which makes the design of the photographing lens unit difficult.
Consequently, it is considered that when the optical part such as the adjustable diaphragm and the ND filter is arranged between the lenses of the photographing lens unit, the entire length of the photographing lens unit becomes longer, but the entire length as the camera module can be shorter. However, the arrangement of the optical part such as the adjustable diaphragm and the ND filter between the lenses increases a lens distance. This deteriorates eccentricity accuracy in front and in rear of the optical member, and gives rise to the need to move the lenses while avoiding the optical member at the time of focusing.
Moreover, the arrangement of the optical part such as the adjustable diaphragm and the ND filter between the lenses of the photographing lens unit results in arranging the lenses while avoiding the adjustable diaphragm and the ND filter. Therefore, an uneven thickness difference, which is a difference between a minimum thickness and a maximum thickness in each of the lenses arranged in front and in rear of the adjustable diaphragm and the ND filter, cannot be secured. In this case, it becomes difficult to eliminate aberrations of the lenses.